They couldn’t arrest him from a civilian law enforcement perspective. A wartime counter terrorism perspective is a different matter.
We were a better country when we were racist enough to round up those of dubious loyalty after being attacked! Now we have ongoing attacks without end! There were no Kamikaze or other attacks here back when we were a racist country! Racism is moral because it protects those like little 8-year-old Martin Richard at the Boston marathon finish line from naturalized Mohammedans!